By CHRISTOPHER TOKIN, M.D.

If you've been suffering from sleepless nights, you may have more than simple insomnia - you may be afraid of the dark.

Results from a new study presented at the Associated Professional Sleep Societies annual meeting in Boston are the first to suggest that some adults can't sleep because they fear darkness.

Researchers in Toronto studied nearly 100 college students who were either good sleepers or poor sleepers. They monitored small twitches in eye muscles during sleep while exposing the subjects to unexpected bursts of sound when the lights were either turned on or off, and what they found was startling.

Poor sleepers were more easily disturbed by the noises, and their reactions were exaggerated by darkness. And over time, while good sleepers became increasingly accustomed to the disturbances, poor sleepers became even more anxious and startled at these alleged bumps in the night.

While treating people with insomnia, Dr. Colleen Carney, principle investigator of the study and director of the Ryeson University sleep and depression laboratory, would continually hear how her patients could only fall asleep if they turned on the television or left the bathroom light on, mannerisms shared with children who are afraid of the dark.

Sure enough, when surveyed, a surprising 46 percent of poor sleepers admitted to harboring current fears of darkness, almost double that observed in good sleepers.

More than 50 percent of Americans report having experienced insomnia in the past year, and 19 percent have chronic sleeping problems.

The high incidence of insomnia among Americans has been attributed to risk factors such as high levels of stress, shift work, or mood disorders such as anxiety or depression. This study, however, is among the first to suggest that an underlying fear of the dark could be a major contributor.

"Listening to unexpected noise is a useful way of assessing fear of the dark because we can't inhibit our startle reflex," Carney said. "And these behaviors are typical of a phobia."

Currently, the National Institute of Health recommends improved sleep hygiene and behavioral therapy as first-line treatments for insomnia. A common recommendation for someone who hasn't fallen asleep after 20 minutes is to do something else away from bed before reattempting sleep. But for someone trying to get over a phobia of the dark, turning the light back on may have the unfortunate effect of making them feel even more awake.

The good news about this is that phobia treatment is one of the big success stories of non-drug therapy, and many frustrated poor sleepers may have finally found a new and easy answer to their problems.